It is well known that sintered silver-cadmium oxide contact members are useful for electrical contact applications. Difficulty has been encountered, however, in both obtaining the desired high densification during sintering, due to the morphology associated with this material, as well as strength and erosion resistance at the operating temperatures of the contacts. In general, such silver-cadmium oxide contact members are prepared by pressing a mixture of silver-cadmium oxide powders into a compact and then sintering the compact by heating to a temperature of about 900.degree. C. for about an hour. Following sintering, in order to further densify the compact, the compact is worked, such as by rolling or pressing.
Particular methods of preparing such contacts include a method such as disclosed in U.S. Pat. No. 3,317,991 wherein single phase silver-cadmium alloy shots, ranging in size from 100 microns to one mm, are formed by atomizing a silver-cadmium alloy. These shots are then subjected to an oxidization treatment to nucleate cadmium oxide particles both within and on the surface of the shots. The oxidized, 2-phase alloy shots are then compacted, sintered and extruded into a desired shape of acceptable density. Due to the use of shots, the oxidation is normally carried out at a high temperature, for example 800.degree. C. for a long period of time. Times ranging from several hours to a day are necessary in order to achieve the desired oxidation due to the bulk of the shots and the slow diffusion of oxygen.
There is another similar process, disclosed in French Pat. No. 1,444,901 wherein a mixture of silver and cadmium oxide, silver oxide and cadmium oxide or silver oxide and cadmium powders is reduced to a homogeneous silver-cadmium alloy powder by heating to 500.degree. C. in hydrogen for two hours. The reduced alloy powder is then oxidized in air or oxygen at a temperature between 400.degree. C. and 650.degree. C. After compacting and sintering, the sintered body is extruded into the desired shape.
There is still another similar proces for preparing silver-cadmium oxide alloy powders as disclosed in U.S. Pat. No. 4,011,052. In this method silver powder of up to 95 micron in diameter and cadmium oxide of up to 2 micron in diameter are blended and subjected to a reduction treatment by heating the mixture in a reducing atmosphere at a temperature between 321.degree. C. and 700.degree. C. The silver-cadmium alloy powder is then sieved and oxidized in air or oxygen by reheating from room temperature to a temperature between 400.degree. C. and 600.degree. C. to oxidize the alloy. The oxidized powder is again sieved, compacted and sintered. The sintered body is then reprocessed to obtain a high density level for the electrical contact application. At a selected time during the process before sintering an oxide of a metal from groups 1A and 2A of the periodic table is added.
The known methods described above, however, do not lead to alloy powders which can be compacted and sintered to an acceptable as-sintered density. Additional costly post-sintering operations such as high pressure coining or hot extrusion have to be performed in order to obtain electrical contact quality materials. The use of a more compactible and sinterable sub-micron size silver powder in any of the methods is either impossible due to impracticality of atomizing into that size range of powder, or coarsening of particles during oxidation to a level of poor sinterability. When a low oxidation temperature below about 550.degree. C. is adopted in combination with the use of sub-micron size silver powder, the resultant powder renders blistered contacts with low as-sintered densities which are susceptible to fracture during subsequent working. Furthermore, the contacts of the materials produced by the known methods do not have optimum erosion resistance during operation. Therefore, the known methods are not designed to work with sub-micron size silver powder, to which the present invention is directed.
The present invention comprises a method of producing an electrical contact using sub-micron size silver powder and cadmium oxide powder, in combination with a low temperature process which avoids undue particle coarsening, to attain blister-free; acceptably high as sintered density compacts having high erosion resistance and ductility.